1. Field of the Invention
The present invention relates to a laminated circuit board (multilayered printed circuit board) provided with copper foil wiring on both sides which are electrically connected by electric conductive material such as conductive metal paste.
2. Description of the Related Art
A conventional laminated circuit board is produced by through hole plating method which is composed of steps of laminating plural multilayered circuit boards, opening drill holes through the whole layers and plating the surface of inner wall of the holes to obtain plating layer for electrical connection.
The advantage of the laminated circuit board made by the through hole plating method is low and stable electric resistance between the respective circuit layers by the plated holes.
However its disadvantage is the complicated process, high manufacturing cost and restriction of applicability of the boards due to the expensive board cost.
Another disadvantage of the plating method is that the holes do not accept electrical parts above them limiting the freedom of parts allocation, and to avoid this, special techniques are proposed such as to drilling the holes slantwise against the board surface.
Recently, this hole plating method has been partly replaced with laminated circuit board having interstitial via holes (IVH) filled with conductive metal paste. Printed circuit board using the conductive paste is more simplified process than the hole plating method thereby achieving lower cost. ALVIH (Any Layer Interstitial Via Hole) board manufactured by Matsushita Electric Co., Ltd is known as an example board applying the paste method.
Due to recent demand for further cost reduction by reducing or shortening of processes however, simultaneous press method has been developed for production of laminated circuit board utilizing conductive paste.
The conductive paste used for laminated board is mainly composed of silver or copper metal powder containing low melting point metals which enable the paste metal soften near at the press temperature for easy bonding resulting in improvement of the stability and time reduction of the production process.
The type and amount of the low melting point metal to be added into the silver or copper paste are determined considering the conductivity level and press temperature during the multilayered circuit board production.
When the laminated circuit board is press formed using the conductive paste containing low melting point metal (hereinafter, referred as conductive paste), however, a diffusion layer of copper and the low melting point metal can sometimes be formed on the copper foil surface which could cause disconnection between the foil and paste due to void and crack generation in the boundary thus seriously deteriorating the connection reliability.